This invention relates generally to methods for operating frequency converters with intermediate DC links, and more particularly, to a method for operating a frequency converter wherein the rotation of a rotating field machine is divided into a plurality of speed ranges, each such speed range having an predetermined associated number of commutations per revolution of the machine.
Siemens-Zeitschrift 45 (1971), pages 753 to 757, and German Pat. No. 22 46 562, teach a method for operating a frequency converter of the type having an intermediate DC link, the inverter having converter valves arranged in a three-phase bridge circuit. The converter supplies electrical energy to a rotating field machine which serves as a load; the rotating field machine being operated at speeds which are higher than a predetermined minimum speed. In the known system, the converter is controlled by the load so that, at speeds above the predetermined minimum speed, the commutating voltage for the converter valves in the inverter are supplied by the rotating field machine. However, at speeds which are slower than the minimum speed, the converter valves of the inverter are commutated by an auxiliary commutating device. In situations where the speed of rotation of the machine is higher than the minimum speed, cyclical commutation occurs six times during each revolution of the machine, the commutations occurring alternately in both halves of the three-phase bridge. Commutation is achieved from a converter valve in a converter branch, to a subsequently current-carrying converter valve of a converter branch in the same bridge half.
In the known converter system, the reactive power for the commutation of the motor current can be obtained from the rotating machine itself at or beyond a predetermined machine operating speed. In the speed range between zero revolutions and the minimum speed which corresponds to approximately one-tenth of the nominal speed of the converter motor, separate commutation aids must be used, particularly during start-up. An additional auxiliary commutating device, or an additional auxiliary start-up commutating device, causes switching of the machine current in the low range of speeds.
The foregoing references teach the manner by which the intermediate link current can be made to have a zero magnitude for a brief period of time by either controlling a rectifier on the network side or by energizing a controlled bypass valve which operates as a switch to short the intermediate link choke. During the brief period of time that the intermediate link current is zero, the current is passed-on to the next machine leg. German Pat. Nos. 22 46 562 and 22 46 592 teach other auxiliary commutating devices by which the intermediate link current is shorted prior to each commutation. The auxiliary commutating systems described therein are suitable for machines having large start-up torques and power ratings.
In systems wherein energy is supplied to rotating-field machines which have high nominal speed, or machines having large numbers of poles, the known auxiliary commutating methods reach their frequency response limit before the necessary minimum speed is reached, and before the load-controlled inverter can commutate the motor current in response to machine voltage. In such cases, other relatively expensive start-up devices, illustratively starter motors or start-up transformers, must be utilized.
It is, therefore, an object of this invention to provide a method for operating a frequency converter of the type having an intermediate DC link in such a manner that the speed range in which forced commutation is supplied does not exceed the frequency limits of conventional auxiliary commutating devices.